Results being returned in multiple rows - sql

Question

I have created a stored procedure that is run at the end of every day which will return the attendance percentage of each student in a class, the result set will show the students previous attendance percentage compared against their new percentage. But for some reason the results are being stored in multiple rows instead of one row. The diagrams below will give you a better understanding of the problem.
The Two Tables:
Student Table
Percentage Table
The code I have written to get the results into a combined table:
select
StudentTb.StudentId,
StudentTb.Forename,
StudentTb.Surname,
CONVERT(VARCHAR(10),DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE())-1, 0),103) as [Previous Reading Date]
case studentTb.Date
when DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE())-1, 0) then PercentageTb.Percentage
End
'Previous Percentage'
CONVERT(VARCHAR,DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()), 0),103) as [Present Reading Date]
case studentTb.Date
when DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE())-1, 0) then PercentageTb.Percentage
End
'Current Percentage'
from studentTb inner join
PercentageTb on studentTb.StudentID = PercentageTb.StudentID
and this is the result set from this query!
However this is not the way I intended the results to look, below shows the desired result set!
I thought the CASE statement would have done that for me but obviously I was wrong, could someone please give me some indication on where I need to go from here?

Answer 1

Why not just do a select with sub select? Assuming a student can only have one result on each day
DECLARE #CurrentDate DATE
DECLARE #PreviousDate DATE
SET #CurrentDate =
SET #PreviousDate =
SELECT
StudentTb.StudentId AS [ID],
StudentTb.Forename AS [Forename],
StudentTb.Surname AS [Surname],
(SELECT Percentage from PercentageTb where StudentID = S.studentID and Date = #PreviousDate) AS [PreviousPercentage]
(SELECT Percentage from PercentageTb where StudentID = S.studentID and Date = #CurrentDate) AS [CurrentPercentage]
from studentTb AS S

Answer 2

One way to accomplish what you want that should work is to use the max() aggregate function to flatten the results like this:
select
s.StudentID,
s.Forename,
s.Surname,
max(case when p.date = CONVERT(VARCHAR,DATEADD(DAY, DATEDIFF(DAY, 1, GETDATE()), 0),103) then p.date end) prev_date,
max(case when p.date = CONVERT(VARCHAR,DATEADD(DAY, DATEDIFF(DAY, 1, GETDATE()), 0),103) then p.percentage end) prev_perc,
max(case when p.date = CONVERT(VARCHAR,DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()), 0),103) then p.date end) curr_date,
max(case when p.date = CONVERT(VARCHAR,DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()), 0),103) then p.percentage end) curr_perc
from studentTb s
inner join percentageTb p on s.StudentID = p.StudentID
group by s.StudentID, s.Forename, s.Surname
Sample SQL Fiddle
Another option would be to join the percentageTb table twice (once for the current date and once for the previous day), see this example.